This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-187902 filed on Jun. 27, 2002.
The present invention relates to a pressure sensor that can be used in low load pressure measurement such as pressure distribution measurement of a human body lying on a bed.
A pressure sensor can be used for acquiring living body information such as respiration and pulse of a sleeper. The living body information of a sleeper can be acquired by detecting the movement of the sleeper. The movement of the sleeper is detected by placing a pressure sensor in an article on which the sleeper lies such as a bed and detecting the variation in the pressure exerted on the pressure sensor. However, the output of the pressure sensor includes the output component corresponding to the load based on the sleeper""s weight, which provides a relatively high signal level, and the output component corresponding to the load variation based on the minute local movement of the sleeper""s body caused by respiration and so on, which provides a relatively low signal level.
Usually, the local movement of the sleeper""s body caused by respiration and so on is so small that the load variation based on the local movement is extremely small than the load based on the sleeper""s weight. Therefore, the pressure sensor is required to precisely detect the load variation based on the minute local movement. Moreover, the pressure sensor is required to make a subject unconscious of the existence of the pressure sensor.
A proposed pressure sensor is disclosed, for example, in the publication JP-A-2001-99726. The proposed sensor includes: a pair of base films; a pair of electrodes located between the base films; a layer of pressure-sensitive resistor located between the electrodes with a predetermined gap from one of the electrodes or two layers of pressure-sensitive resistor respectively located on the electrodes with a predetermined gap; and a spacer located outside the layer(s) of the pressure-sensitive resistor to form the gap. The contact state between the layer(s) of the pressure-sensitive resistor and the electrodes varies in response to the pressure exerted on the pressure-sensitive resistor through the base films. That is, the pressure is detected on the basis of the variation in the resistance between the electrodes.
In the proposed pressure sensor mentioned above, the base films and the spacer are thinned to permit the contact state between the layer(s) of the pressure-sensitive resistor and the electrodes to vary readily. Therefore, it is possible to precisely detect the load variation based on the minute local movement and to make a subject unconscious of the existence of the pressure sensor.
When living body information such as respiration and pulse of a sleeper is detected using a pressure sensor, a pressure sensor sheet that is made up of a plurality of the proposed pressure sensors has usually been used. In the pressure sensor sheet, the base films of the sensor are commonly used. That is, the plurality of the proposed pressure sensors are formed by arranging electrodes, pressure-sensitive resistors, spacers, and so on between a pair of large base films. With the arrangement, it is possible to stably detect the living body information even if the position of a sleeper varies on the pressure sensor sheet when the sleeper turns.
However, there is a problem with the proposed pressure sensor that the output deviation between the sensors and the output deviation in repeated detections increase because the base films are thinned are thinned in order to decrease the detectable lowest pressure. In addition, the signal level of the sensor output can be affected when the portions of the base films outside the spacers are stretched by the sleeper or the article on which the sleeper lies. As a result, it becomes difficult to maintain a good detection precision. This is especially prominent in the proposed pressure sensor. That is, there is a problem with the proposed pressure sensor that as the detectable lowest pressure decreases, it becomes difficult to maintain a good detection precision.
The present invention has been made in view of the above aspects with an object to provide a pressure sensor that can be used for a pressure sensor sheet to detect a low pressure with preferably high detection precision.
To achieve the above object, a pressure sensor according to the present invention includes a pair of base films, a pair of electrodes, a layer of pressure-sensitive resistor, a spacer, and a projection. The electrodes are located between the base films. The layer of pressure-sensitive resistor is located between the base films to be distant from one of the electrodes by a predetermined gap. The spacer is located outside the layer of pressure-sensitive resistor between the base films and used for forming the gap. The projection is located on an outer surface of one of the base films in order to decrease the lowest pressure that can be detected by the pressure sensor. The contact state between the layer of pressure-sensitive resistor and one of the electrodes varies to vary the resistance between the electrodes when a pressure acts on the projection.